The invention described herein relates to a normally-closed two-port directional needle-valve, controlled by solenoid either directly, or indirectly, in which fluid under pressure is able to enter radially and exit axially or, alternatively, enter axially and exit radially.
The singular feature of this valve is embodied in the fact that the needle-obturator draws away from its relative seating to a generous distance--even using a quite low-power magnet and pressurizing fluid to some 400-bar--with the result that flow ports in the valve may range from a diameter of 1 mm for the direct-acting type up to 20 mm and more for piloted versions.
Valves of the type aforesaid in current use make provision, in basic terms, for a movable core able to travel axially and be attracted thus by a fixed core, the assembly formed by the two being ensheathed by a winding through which electric current may pass; the movable core incorporating a needle-type obturator serving to close off a fluid flow-port, and a spring compressed so as to maintain a given distance between the movable core and the fixed core.
In the case of normally-closed valves, this spring will be noticably weak, in that it serves merely to ensure engagement of the needle-obturator in its respective seating. On the other hand, once the same obturator is duly seated it falls under the thrust of fluid pressure, in consequence of which the effort produced by the solenoid must be sufficient to overcome both the strength of the spring and that of the value equivalent to: seating-section area multiplied by fluid pressure. What in fact happens is that the force with which the movable core is attracted by the fixed core becomes inversely proportionate to the distance existing therebetween--hence the maximum force of attraction between the two cores comes about once the movable core lies close as can be to the fixed core, wherefrom it will be clear that if one is seeking libera1 distances on separation, the need automatically arises for solenoids of some considerable power. The drawback thus outlined imposes serious limitations on the use of this type of solenoid valve for flow rate in excess of 15 liters per minute and with fluid pressure higher than 250-bar, in that the use of overlarge solenoid units would create enormous problems with regard to high input current and subsequent overheating of their coils, not to mention greater overall dimensions and the unacceptable cost increase. Another way of tackling the problem is to provide for the needle-obturator's open-stroke being limited to a few tenths of a millimeter (in such a way that the attractive force between fixed core and movable core remains markedly strong, given the closeness of the two)--although here one has other significant disadvantages--viz, considerable loss of fluid energy through choking-up of the outlet port and, worse still, problems in construction arising from extremely tight machining tolerances, perhaps leading to piece-by-piece adjustment of the valve's single components. At all events, one is left with the snag of unobtainable high flow-rates.
The overriding object of the invention described herein is that of allowing for the use of ultra-low power solenoids in conjunction with release-strokes of length such as to put as such as 3 or 3.5 mm between needle and seating upon opening of the obturator, even where fluid being checked might be pressurized to as much as 400-bar. Numerous advantages derive from this combination, amongst which the facility of holding the solenoid on-current for unlimited periods of time by virtue of its low power-consumption; reduced dimensions and significantly lower production costs; more generous machining tolerances, with no need for fine adjustments from valve to valve; handling of much higher flow-rates without significant loss of fluid power.